When a drug is administered into the body, only a small amount of the drug may reach its target site and most of the administered dose is distributed to non-targeted sites and may cause undesirable side effects. Therefore, in the last two decades, research has focused on the development of systems efficient for site specific delivery of drugs by the use of appropriate carriers, which include liposomes, small molecular surfactant micelles, polymeric nanoparticles, and polymeric micelles (polymeric nanoparticles made of hardened micelles). The use of liposomes as drug carriers has been found to be limited mainly due to such problems as low entrapment efficiency, drug instability, rapid drug leakage, and poor storage stability. Small molecular surfactant micelles are easily dissociated when they are diluted in body fluids after been administered into the body, and therefore it is difficult for them to perform sufficiently as drug carriers.
Recently, polymeric nanoparticles and polymeric micelles using biodegradable polymers have been reported to be extremely useful technologies for overcoming these problems. They change the in vivo distribution of an intravenously administered drug thereby reducing its side effects and improving its efficacy thereby offering such advantages as cell specific targeting and control of the release of the drug. They also have good compatibility with body fluids and improve the solubility and bioavailability of poorly water-soluble drugs.
Nanometer size drug carriers with hydrophilic surfaces have been found to evade recognition and uptake by the reticulo-endothelial systems(RES), and thus to circulate in the blood for a long period of time. Another advantage of these hydrophilic nanoparticles is that, due to their extremely small size, the particles extravagate at the pathological sites, such as solid tumors, through a passive targeting mechanism. However, successful drug delivery to the specific target site requires stable retention of the drug by a carrier while in the circulation. Since drug targeting appears to require a long circulation time and the carrier is exposed to blood components for a long period of time, the stability of a drug-carrier association needs to be improved over that of carriers that are rapidly cleared.
Among the nanometer size drug carriers with hydrophilic surfaces, polymeric micelles usually consist of several hundreds of block copolymers and have a diameter of about 20 nm to 50 nm. The polymeric micelles have two spherical co-centric regions, a densely packed core of hydrophobic material which is responsible for entrapping the hydrophobic drug, and an outer shell made of hydrophilic material for evasion of the body's RES which permits circulation in the blood for a longer period of time. In spite of their distinct advantages such as small size, high solubility, simple sterilization, and controlled drug release, the physical stability of these carriers is a critical issue since the rapid release of the incorporated drug may occur in vivo.
Micelles are thermodynamically stable if the total copolymer concentration is above the critical micelle concentration (CMC). Thus, the use of a copolymer system with a low CMC value may increase the in vivo stability of the micelles. The kinetic stability means the rate of disassembly of a micelle. The rate of disassembly depends upon the physical state of the micelle core. Micelles formed from copolymers containing a hydrophobic block which has a high glass transition temperature will tend to disassemble more slowly than those with a low glass transition temperature. They are also likely to be affected by many of the same factors that affect the rate of unimer exchange between micelles. The unimer exchange rate has been found to be dependent on many factors such as the content of solvent within the core, the hydrophobic content of the copolymer, and the lengths of both the hydrophilic and hydrophobic blocks.
Great efforts have been devoted to the development of a biodegradable and biocompatible core-shell type drug carrier with improved stability and efficacy, which will entrap a poorly water-soluble drug. A preparation method of chemically fixing polymeric micelles, wherein the polymer is a core-shell type polymer comprising a hydrophilic polyethylene oxide as the shell and a hydrophobic biodegradable polymer that is cross-linked in an aqueous solution as the core, was disclosed in EP 0,552,802A2. However, these polymeric micelles are difficult to prepare because a cross linker must be introduced into the hydrophobic component of the A-B type di-block or A-B-A type tri-block copolymer so that the core-forming polymer has a stable structure. Also, using a cross linker that has never been used before in a human body raises safety concerns.
A micelle forming block copolymer-drug complex was disclosed in U.S. Pat. No. 6,080,396. The high molecular block copolymer-drug complex in which the high molecular weight block copolymer, having a hydrophilic polymer segment and a hydrophobic polymer segment, forms a micelle having the hydrophilic segment as its outer shell and contains an anthracycline anticancer agent in its hydrophobic inner core. The molecules of the anticancer agent are covalently linked within the micellar core. However, when the drug is covalently linked within the polymeric micelles, it is difficult to control the cleavage rate of the drug-copolymer linkage.
On the other hand, a report shows that the solubilization of a hydrophobic drug can be achieved by a polymeric micelle composed of a di- or tri-block copolymers comprising a hydrophilic polymer of polyalkylene glycol derivatives and a hydrophobic biodegradable polymer such as fatty acid polyesters or polyamino acids. U.S. Pat. No. 5,449,513 discloses a di-block copolymer comprising polyethylene glycol as the hydrophilic polymer, and a polyamino acid derivative, e.g. polybenzyl aspartic acid, etc., as the hydrophobic polymer. This di-block copolymer can solubilize hydrophobic anticancer agents, e.g. doxorubicin, or anti-inflammatory agents, e.g. indomethacin. However, the polyamino acid derivative cannot be hydrolyzed in vivo, and thus causes side effects due to immune responses that are excited
One approach to improve the stability of polymeric micelles is to increase the hydrophobicity of the polymer. To do so, the molecular weight or the concentration of the polymer should be adjusted. However, as the molecular weight is increased, its biodegradability is decreased, and so the polymer is poorly excreted from the body and accumulates in organs causing toxic effects therein. U.S. Pat. No. 5,429,826 discloses a di- or multi-block copolymer comprising a hydrophilic polyalkylene glycol and a hydrophobic polylactic acid. Specifically, this patent describes a method of stabilizing polymeric micelles by micellizing a di- or multi-block copolymer wherein an acrylic acid derivative is bonded to a terminal group of the di- or multi-block copolymer, and then, in an aqueous solution, the polymer is crosslinked in order to form the micelles. The above method could accomplish stabilization of the polymeric micelle, but the crosslinked polymer is not degraded, and thus, cannot be applied for in vivo use. The above polymeric micelles can solubilize a large amount of poorly water-soluble drug in an aqueous solution with a neutral pH, but the drawback a that the drug is released within a short period of time. Also, in U.S. Pat. No. 6,458,373, a poorly water-soluble drug is solubilized into the form of an emulsion with α-tocopherol. According to this patent, to stabilize the emulsion, PEGylated vitamin E is used as a surfactant. PEGylated vitamin E has a similar structure to the amphiphilic block copolymer comprised of a hydrophilic block and a hydrophobic block, and the highly hydrophobic tocopherol increases the copolymer's affinity with a poorly water-soluble drug, and thus, it can solubilize the poorly water-soluble drug. However, polyethylene glycol used as the hydrophilic polymer has a limited molecular weight, and so PEGylated vitamin E alone can solubilize a hydrophobic drug such as paclitaxel only up to 2.5 mg/ml. At 2.5 mg/ml or more, unstable micelles are formed, and the drug crystals are likely to form precipitates.
Clinical tumor resistance to chemotherapy can be inherent or acquired. Inherent resistance is present in the tumors that fail to respond to the first-line chemotherapy at the time of diagnosis. Acquired resistance occurs in the tumors that are often highly responsive to the initial treatment, but on recurrence, exhibit an entirely different phenotype. The resistance can be formed to both previously used drugs and new drugs with different structures and mechanisms of action. For example, cancer chemotherapy with Taxol® often fails due to the acquired resistance of cancer cells, which is frequently associated with the overexpression of P-gp and alteration of β-tubulin. Taxol® resistant cells exhibit cross-resistance to other drugs including actinomycin D, doxorubicin, vinblastine, and vincristine. Therefore, clinical drug resistance is a major barrier to be overcome before chemotherapy can be curative for patients with metastatic cancer.
Drug-resistant cancer cells show a higher a IC50 (50% cell inhibition concentration of drug), and so for chemotherapy to be effective a higher concentration of drugs is needed for the tumor cells while reduced drug concentration is desired for the normal cells. Therefore, longer systemic circulation and specific localization of drugs in the tumor tissues are required for improving the effectiveness against drug-resistant cancers.
In view of the foregoing, the development of an improved polymeric micelle composition for hydrophobic drug delivery that is biocompatible and biodegradable has been appreciated and desired. The present invention provides such an improved polymeric micelle composition which is biocompatible and biodegradable, and can effectively deliver a hydrophobic drug without a decrease in its stability.